We propose to design a low noise, two-side buttable, 64x64 readout multiplexer with the following key design features: 1- By far the largest readout array developed for far IR detectors to date. Four of these readout can be butted together to form a >16k-pixel mosaic array satisfying the need of the next generation of astronomical instruments. 2- Optimized for use with far infrared detectors requiring low bias levels. The unit-cell design will maintain constant bias across the detector during the integration eliminating non-linearity and detector debiasing. The design will also minimize pixel-to-pixel DC variations which improves the bias uniformity across all pixels of the array. 3- Capable of operation at cryogenic temperatures at least as low as 1.6K. Advanced monolithic cryo-CMOS technology will guarantee deep cryogenic operation with minimal impact on noise performance. 4- Offers the potential of being directly hybridized to IR detector arrays using planar bump-bond technology. This technology has been identified by NASA as well as the science and astronomy community as key for future far IR astronomy. It fits well within the scope of the SBIR Subtopic S1.04 and will be a benefit to many large and small NASA missions including SAFIR/CALISTO and SOFIA.